Friction welding machines are generally well known in the art. In a friction weld, heat is generated by rubbing two workpieces together until the material at the interface between the two pieces reaches a plastic state. The two workpieces are then forged together under pressure to finalize the weld and expel gases, thus forming a single component having an integral bond. A friction weld can typically be formed in a very short period of time compared to more conventional are welding methods, and thus friction welds are less labor intensive, more uniform and more cost effective than conventional methods. Friction welders are especially well suited for welding round bars or tubes to each other, or for welding round workpieces to flat plates, disks or gears. The friction welding process is frequently used to produce automotive drive shafts, automotive air bag canisters, gear shafts and engine valves, as well as other applications in which a high quality weld is required.
On a friction welder, one of the workpieces is mounted to a rotating chuck assembly while the other workpiece is fixed in a stationary chuck or tailstock. A drive motor accelerates the rotating chuck to a predetermined speed, and the parts are then forced together with the friction induced heat producing a material flux. The pieces are then forged together under pressure, which expels gas and produces a fine grain weld.
Friction welders are generally divided into two categories, inertia friction welders and the more conventional direct drive friction welders. The rotating chuck on inertia friction welders is drivingly connected to a flywheel. A drive motor accelerates the flywheel to speed, the drive motor is then disconnected, and the kinetic energy stored in the flywheel is converted to heat energy as the two workpieces are forced together under extremely high pressure. The rotating chuck rapidly decelerates to a stop and the weld is formed at the interface between the two workpieces. Inertia friction welding has a number of inherent drawbacks which makes it unsuitable for many applications. First, the flywheel bearings gradually heat up, which depletes the available kinetic energy because energy is lost through increased friction. Second, due to the very high forge pressure required, inertia welding is unsuitable for thin walled tubes and many soft metals, such as aluminum. In general, the quality and uniformity of inertia friction welds are hard to control.
On direct drive friction welders, the drive motor used to rotate one of the workpieces remains engaged until the weld is complete and the rotating workpiece comes to a halt. Unfortunately, the final orientation of the rotating workpiece relative to the stationary workpiece is not easily controlled. In many applications, it is critical that the two workpieces be welded together in a predetermined angular orientation relative to one another. For example, the yoke at one end of an automotive drive shaft must be perpendicular to the yoke at the other end of the drive shaft; otherwise, the drive line components will be prone to premature failure. Similarly, on many gear shafts and other components the gear at one end of the shaft must be precisely located relative to another gear or cam lobe on the shaft.
In order to achieve precise angular orientation a number of approaches have been attempted. For example, one prior art approach uses a defined braking mechanism which applies a braking force as the rotating spindle decelerates and approaches the desired final orientation which in turn is conveyed to the rotating spindle via an electronic signal or mark. Usually however, due to variations in the braking mechanism and other variables, the deceleration of the spindle is not uniform. Frequently, the brake must be released and the drive motor must be temporarily re-engaged in order to force the spindle to the desired location. Thus the rate of deceleration, as well as the final angular position of the rotating workpiece, is relatively uncontrolled. In many instances as the spindle approaches the desired stopping point, it becomes clear that the spindle has or will stop short of the desired alignment mark, while at other times the spindle will completely pass the mark. If the spindle stops short, the drive motor is re-engaged and the spindle is accelerated and driven to the mark. If the spindle overshoots the mark, the drive motor is re-engaged and the spindle is turned an extra rotation in order to reach the mark again. Unfortunately, in each instance the spindle has slowed significantly and the weld has already begun to cool and the material has begun to harden. Any subsequent accelerations and rotations of the spindle cause microfractures in the crystal structure of the material, resulting in a lower quality, high risk weld. Furthermore, the defined braking method is not accurate enough for many applications. In general, the defined braking method is unsuitable for applications in which the final angular orientation is critical and is also unsuitable for many aluminum welds, aircraft quality welds, air bag welds and other safety related welds.
Accordingly, there exists a need for a control system for friction welders that can control the final orientation of one workpiece relative to the other, and that consistently produces a uniform high quality weld suitable for use on aircraft and safety related applications and on a wide variety of material types.